Removing reverb circuit from JM Signature amp.

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Guy77
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Removing reverb circuit from JM Signature amp.

Post by Guy77 »

Hi, I wanted to build a JM Signature amp without the reverb circuit. I have attached a copy of the JM signature schematic and have drawn where I would remove the reverb circuit. By removing the reverb I was hoping to build the amp with only 2 pre-amp tubes as opposed to 5.
I am currently finishing up a 100 watt version of the amp with reverb. Will post pics when done in next few weeks. I wanted to make one with no reverb after the 100 watter is done.

Any input appreciated.

Cheers
Guy
This is original with reverb.
JM SigWith_Reverb.jpeg
This is without reverb
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Last edited by Guy77 on Mon Nov 06, 2017 10:44 pm, edited 3 times in total.
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Re: Removing reverb circuit from JM Signature amp.

Post by pompeiisneaks »

Looks to me like you're removing V4B that is part of the preamp side 'mixing' section, not sure if that's needed or not, but usually you'd leave that one in, and ground the other side but retain that 68k from the reverb section. I don't know how much of an impact that extra triode will have if you remove it.

As is, the clean signal goes through that stage and removing it will change the sound.

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M Fowler
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Re: Removing reverb circuit from JM Signature amp.

Post by M Fowler »

It's a two tube reverb remove 2A/b and 3A/b
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Re: Removing reverb circuit from JM Signature amp.

Post by pompeiisneaks »

Except that V4A does nothing in the circuit at that point and isn't needed right? It's the 'mixer' stage of that side of the signal path.

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Guy77
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Re: Removing reverb circuit from JM Signature amp.

Post by Guy77 »

Thanks Phil and Mark for the quick replies! I will remove v2AB and v3AB. And yes it seems to make sense to just keep v4B and not v4A.
Also Phil when you say to keep the 68K resistor are you referring to the one I circled here in the attached layout in orange, on the right side of board? Also I believe I will need to leave the 1M resistor that comes before that 68K resistor that I circled in orange.

Cheers!
Guy
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sluckey
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Re: Removing reverb circuit from JM Signature amp.

Post by sluckey »

The 68K he's referring to is the one that connects to V4A pin 1. You can remove that.
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Guy77
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Re: Removing reverb circuit from JM Signature amp.

Post by Guy77 »

Thanks Sluckey. I see that one now. So you are saying I can remove that 68K, ok thanks.
I forgot how many 68k resistors were in this build! I will start on this build once I finish my 100 watt JM Signature in a few weeks and post my results.

Cheers!
Guy
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Re: Removing reverb circuit from JM Signature amp.

Post by sluckey »

Since you are building another Signature with reverb you could try the idea you showed in the pdf file, ie totally eliminate V4. It would be an easily reversible mod. Then you could decide if you like the sound of the amp with just a two stage preamp. There are plenty of amps with this gain structure. You would not need to actually disconnect the reverb circuit. Just pull V2, V3, and V4.
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Re: Removing reverb circuit from JM Signature amp.

Post by pompeiisneaks »

So sluckey I was wondering, isn't that going to adjust the gain structure of that amp though? Then he's no longer got a JM Signature channel, or does the mixing of the two need that to re-balance the loss in the mixing itself?

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Re: Removing reverb circuit from JM Signature amp.

Post by norburybrook »

Phil,

I was thinking the same thing earlier in this thread. The reverb is what makes this amp what it is, otherwise isn't it just a classic tonestasck dumble ODS with no overdrive?


....A small special in other words.


Marcus
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Re: Removing reverb circuit from JM Signature amp.

Post by sluckey »

isn't that going to adjust the gain structure of that amp though?
Sure. Losing the V4b circuit will lower the gain. But removing that 1M and 68K from the V1b circuit will increase the gain. Is the increase enough to offset the loss of V4b? I don't know. That's why I suggested he try this mod on the amp he's currently building. Then he will know what it sounds like and decide which way he wants to proceed with the next reverb-less build.

Keeping the V4-b circuit should sound about the same as the reverb amp with the reverb simply turned off. But he may like the circuit the other way. He has more than one option.
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Re: Removing reverb circuit from JM Signature amp.

Post by pompeiisneaks »

sluckey wrote: Tue Nov 07, 2017 5:17 pm
isn't that going to adjust the gain structure of that amp though?
Sure. Losing the V4b circuit will lower the gain. But removing that 1M and 68K from the V1b circuit will increase the gain. Is the increase enough to offset the loss of V4b? I don't know. That's why I suggested he try this mod on the amp he's currently building. Then he will know what it sounds like and decide which way he wants to proceed with the next reverb-less build.

Keeping the V4-b circuit should sound about the same as the reverb amp with the reverb simply turned off. But he may like the circuit the other way. He has more than one option.
Gotcha, perfect, thanks!

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Guy77
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Re: Removing reverb circuit from JM Signature amp.

Post by Guy77 »

Thanks again Sluckey! I will make the changes as you mentioned and pull V2,V3 and V4 tubes. I will get the amp working with reverb first before I make the changes. Should be done building in the next 2 - 3 weeks.

I will report back with results on how it sounds.

Cheers!

Guy
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Guy77
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Re: Removing reverb circuit from JM Signature amp.

Post by Guy77 »

I finally got around to completing this test of removing v2 v3 and v4 from my JM 100 amp and running the signal as illustrated here.
JMsig.png
I then went back and setup the amp in stock form with signal going through the 1M resistor and then through v4b ( reverb turned off)

The tone that I like so much about the JM amp is gone once v4 and the 1M resistor is removed from the circuit.
The cleans were sweeter and rounder sounding and had more snap! I know it seems strange but for some reason it just sounds better at least to my ears.

Happy New Year everyone.

Cheers

Guy
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Re: Removing reverb circuit from JM Signature amp.

Post by norburybrook »

It is a sweet, round sounding amp for sure :D It makes a lovely jazz guitar amp too.


Happy new year all.



Marcus
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